(1) Field of the Invention
This invention relates generally to encoding of video information and, more particularly, to predictive encoding using motion compensation.
(2) Description of the Prior Art
In U.S. Pat. No. 3,632,865 issued to B. G. Haskell et al on Jan. 4, 1972, it was recognized that television scenes containing objects moving more or less in translation could be more efficiently encoded by computing and then encoding the intensity differences between picture elements in the moving area with respect to elements in the previous frame which are appropriately translated or displaced. In a similar manner, differences could be constructed from linear combinations of picture elements. Another reference describing motion compensated coding is "Television Bandwidth Compression Utilizing Frame to Frame Correlation and Movement Compensation", F. Rocca, Symposium on Picture Bandwidth Compression, (MIT, Cambridge, Mass., 1969) Gordon and Breach, 1972.
In the Haskell system, two types of picture areas are identified, namely, moving areas and background. The picture information is encoded by identifying (1) the address of the picture elements in the moving area, (2) the average amount of translation for a group of pels in the moving area, and (3) prediction error values for the moving area pels. While the Haskell et al approach is successful in increasing the encoder efficiency, several problems nevertheless remained. First, estimation of displacement required a large number of complex calculations, and the estimate was not accurate when a large number of moving objects existed, or when different parts of an object moved differently. Second, the encoder did not adequately distinguish between truly moving objects and uncovered background areas, thereby wasting an opportunity for still further efficiency increases.
Many of the above-mentioned problems with displacement estimation are overcome by the invention described in applicants' concurrently filed application Ser. No. 021063 entitled "Technique for Estimation of Displacement and/or Velocity of Objects in Television Scenes". In accordance with that invention, displacement is estimated recursively, using the previous estimate to form successive updates or correction terms. However, since the estimates are generally updated on a pel by pel basis, applicants' estimation apparatus is not readily compatible with the prior art encoder of Haskell, because that encoder requires a single displacement estimate per field, frame or block of data.
In view of the foregoing, it is the broad object of the present invention to improve the efficiency of motion compensated coding of video signals, using improved motion estimation techniques. Specific objects are to provide a coder which can distinguish between truly moving areas and uncovered background and which can compensate adequately in the case of multiple moving objects. Naturally, the encoder must be readily implemented without unduly complex or expensive circuitry.